This invention relates generally to the field of integrated optoelectronics, and more particularly, to a vertical cavity surface emitting lair integrated with electrically or optically sensitive interactive components, such as a photodetector, a transistor, or diode. One embodiment of the invention is an optical-input/optical-output device in either a vertical or side-by-side geometry suitable for use in one- and two-dimensional arrays and as individual elements in optoelectronic integrated circuits (OEICS). Another embodiment of the invention is an electrical-input/optical output device in either a vertical or side-by-side geometry suitable for use in one- and two-dimensional arrays.
Devices integrating photodetector and photoemitting components with light-emitting components are known, and the following references discuss multilayer devices or combinations of devices that receive optical inputs, convert them to electrical signals, and then use the electrical signal to cause light emission. These particular devices are prone to latch-up, that is, although the devices can be turned on with a light pulse, they can't be turned off without significantly decreasing the electrical current flowing through the device because of either optical or electrical excessive positive feedback. This is viewed as a disadvantage for a system where all signals are optical. Photodiodes are inefficient in converting electrical power to optical power and are not as directional as lairs. Edge-emitting lasers are not readily fabricated into two-dimensional arrays.
The reference of Beneking, H., GaAs-GaAlAs phototransistor/laser light amplifier, ELECTRON. LETT. 16(15): 602-603, 1980 July 17, discusses the integration of phototransistors on top of edge-emitting lasers tier use as a light amplifier. Beneking, H. Full Solid State Image Converter Based on Integration of Phototransistors and light-emitting diodes, ELEC. DEV. LETT. EDL-2(4): 99-100, 1981 April 4, discusses arrays of phototransistors and light-emitting diodes one on top of the other for purposes of converting an image from one wavelength to another.
Sasaki, A. and M. Kuzuhara, in GaAsP-InP Heterojunction Phototransistors and Light Amplifiers, JAPAN J. OF APPL. PHYS. 20(4): L283-L286, 1981 April discusses the vertical integration of a phototransistor and a light-emitting diode. Although the configuration disclosed prevents electrical feedback to avoid latching operation, it does not discuss optical feedback which would indeed cause the device to latch.
Taylor, G. W., J. G. Simmons, A. Y. Cho and R. S. Mand, A new double heterostructure optoelectronic switching device using molecular beam epitaxy, J. APPL. PHYS. 59(2): 596-600, 1986 Jan 15, discuses a multilayer device closely related to p-n-p-n structures such as thyristors to be used for optical logic and memory. In some ways these devices can be thought of as an light-emitting diode (p-n junction) placed next to a phototransistor, a n-p-n device, but if these devices are connected in such a way that both electrons and holes flow between them, they latch-up when turned on. Taylor also specifically mentions that while his devices emit spontaneous emission, like that from light-emitting diodes, they could be made to lase (stimulated emission) by configuring the layers in a cavity. The reference, however, does not provide the teaching for the implementation of either the edge-emitting or the vertical cavity surface emitting type lasers.
Kasahara. K., Y. Tashiro, N. Hamao, M. Sugimoto and T. Yanase, Double heterostructure optoelectronic switch as a dynamic memory with low-power consumption, APPL. PHYS. LETT. 52(9): 679-681, 1988 Feb 29, addresses making electrical contacts to interior layers of a p-n-p-n device like Taylor's above, in order to improve the electrical switching speed and increase flexibility.
Tashiro, Y., N. Hamao, M. Sugimoto, N. Takado, S. Asada, K. Kasahara and T. Yanase, Vertical to surface transmission electrophotonic device with selectable output light channels, APPL. PHYS. LETT. 54(4): 329-331, 1989 Jan 23, discusses a p-n-p-n layered device formed into an edge-emitting laser with multiple outputs. It also presents laser emission out of the surface of the wafer by turning the emission from the edge of lasers using integral 45.degree. mirrors. This approach has the disadvantage of requiting more surface area on the wafer and requiring vertical facets formed either by cleaving or sophisticated dry etching techniques.
Lin, S. H., J. H. Kim, J. Katz and D. Psaltis, integration of high-gain double heterojunction GaAs bipolar transistors with a light-emitting diode for optical neural network application, IEEE/CORNELL CONFERENCE ON ADVANCED CONCEPTS IN HIGH SPEED SEMICONDUCTOR DEVICES AND CIRCUITS, 344-352, 1989, addresses integrating multiple transistors and phototransistors with light-emitting diodes in arrays for neural network applications. The devices are laid out side-by-side which is required for the multiple electrical connections when more than one transistor is used to drive the light-emitting diode.
U.S. Pat. No. 4,891,093, entitled "Processes For the Manufacture of Laser Including Monolithically Integrated Planar Devices," to Smith, Jan. 2, 1990, discloses a method and a structure suitable for monolithic integration of an edge-emitting laser and another device, e.g. a field-effect transistor in a laterally offset planar region. Although Smith discloses a laser monolithically integrated with other devices capable of driving the laser, the actual laser used is an edge-emitting laser. The method disclosed in Smith '093, moreover, is most applicable to planar devices requiring low doping concentrations, e.g. the field-effect transistor, and the devices are positioned laterally.
U.S. Pat. No. 4,910,571, entitled "Optical Semiconductor Device," Kasahara et al., Mar. 20, 1990, teaches an optical semiconductor device for receiving and emitting light at laterally separated points to facilitate alignment of light axes for multiple inputs or outputs. Although the device disclosed in the patent reference has optical inputs and optical outputs, and uses layers of semiconductor materials that have an overall doping structure of p-n-p-n, its purpose is to provide a plurality of laterally separated regions of light reception and emission, instead of a single receiving and single outputting region that are laterally coincident. The device of Kasahara '571 requires that the light inputs and outputs be on the same side of the substrate and produces optical output from a light-emitting diode, which is an incoherent light source. The device of Kasahara '571 latches so that it emits light even after the input light is removed until the electrical supply is removed.
U.S. Pat. No. 4,879,250, entitled "Method of Making a Monolithic Interleaved light-emitting diode/PIN Photodetector Array," to Chan, Nov. 7, 1989 discloses arrays of different types photoelectric devices (e.g. photodiodes and light-emitting diodes) fabricated from the same epitaxial layers, but these arrays have light inputs and outputs on the same side of the substrate. The photodiodes provide no electrical gain, are inefficient in electronic to optical conversion, and are not as directional as lasers.
U.S. Pat. No. 4,833,511, entitled "Phototransistor Device," to Sugimoto, May 23, 1989, discloses an optically controllable device where the level of the input signal changes the absorption in the device and hence the amount of transmitted light. The device of Sugimoto '511 controls optical output signals with optical input signals, and uses optical signal paths perpendicular to the substrate, and use phototransistors to amplify the current generated by low levels of input signal. Sugimoto's device, however, contains no emitting element for converting electrical signals into optical signals. The light output is always smaller than the light input because the output is just the unabsorbed portion of the input. Moreover, the device has as an object of its design a bistable or hysteretic behavior.
U.S. Pat. No. 4,888,625, entitled "Optoelectronic Coupling Element, and Method of Making Same," to Mueller, Dec. 19, 1989 discloses an optocoupler of solid construction formed by affixing a chip of light emitting elements to one side of an optic coupling medium and a chip of light detecting elements to the other. The light emitting elements are intended to direct their light output to the light detecting elements attached to the same piece of optic coupling medium. It takes an electrical input, converts it to light which passes through the optic coupling medium to a detector which converts the light to electricity. While the device of Mueller '625 uses light emitting and detecting elements which direct their light perpendicular to the plane of the devices, and uses stacked geometries with the two elements at the same lateral point, its purpose is to convert an input electrical signal to an output electrical signal using elements that are optically connected, rather than acting upon an input signal to produce an output optical signal using elements that are electrically connected.
U.S. Pat. No. 4,947,400. entitled "Laser-Photodetector Assemblage," to Dutta, Aug. 7, 1990, discloses a monolithic integration of an edge-emitting laser and a phototransistor situated side-by-side so that the phototransistor responds to light leaking out of the laser. The electrical signal from the phototransistor is used as a monitor of the laser power. The use of the phototransistor, in Dutta '400, is to sense the output of a laser, rather than to control the lair. The geometry of these devices is also a side-by-side arrangement.
The operation of the heterojunction phototransistor as an individual component has been described in the literature, see e.g., J. C. Campbell, Phototransistors for Lightwave Communication, in SEMICONDUCTORS AND SEMIMETALS, Vol. 22, Part 2, pp. 389-447 (1985); and the operation of the vertical cavity surface emitting laser as an individual component was described in Jewell. J. L. et al., Low-Threshold Electrically Pumped Vertical-Cavity Surface-Emitting Microlasers, ELECT. LETT., Vol. 25, No. 17, Aug. 17, 1989, pp. 1123-1124.
It is thus an object of the invention to provide an optical inputs/optical outputs or electrical inputs/optical outputs device suitable for use in arrays, and as individual elements in optoelectronic integrated circuits. This object is achieved by the integration of a photodetector, transistor, diode, or other photo- or electro-active components with a vertical cavity surface emitting laser in a either a vertical or a side-by-side arrangement. The vertical and the side-by-side geometry provides for small, compact, and densely-spaced arrays and individual elements in optoelectronic integrated circuits (OEICS) which enable, for example, optoelectronic interconnects, laser-scanning printing and projection, optical communication, neural networks, and optical computing. Additionally, the geometry simplifies the required electrical contacts between the components.
It is yet another object of the invention to provide an integrated optoelectronic device with high optical gain. In the photodetector/vertical cavity surface emitting laser embodiment, this object is achieved by either using a photodetector with intrinsic electronic gain, such as a phototransistor, or by using a photodetector such as a PIN photodiode in combination with a separate component, such as an electronic amplifier. The vertical cavity surface emitting laser further contributes to the increased optical gain by having efficient electronic-to-photonic conversion and lasing action. The mulitmilliwatt optical output of the vertical cavity surface emitting laser advantageously enables a fan-out capability because the light can be split and redirected. In addition, cascadability can be achieved if the output of the vertical cavity surface emitting laser or the output interactive component drives a similar but separate device. Cascadability facilitates optical computing and neural networks. Additional advantages achieved by the particular components is that the device is less sensitive to small fluctuations in power, and the device is much faster because more power is available to drive the next device.
Yet, still another object of the invention to provide a fundamental optical device characterized by high contrast and stability. High contrast is the ratio of the maximum optical output power of the device turned on to the power of the light emitted when the device is just below threshold for turn on. The feature of the invention that achieves this object is that the vertical cavity surface emitting laser has a threshold current, and the nonlinear response of the vertical cavity surface emitting laser to current input. In addition, an interactive component such as a phototransistor, may be designed for a nonlinear response. These features of the invention enable compatibility with digital or sigmoidal (smooth) switching applications. Moreover, the device is less susceptible to noise or fluctuations in the supply voltage, or in optical or electronic fluctuations in individual components of the device. Specific applications requiring a nonlinear response, such as neural network and sigmoidal switching applications, are also available.
It is yet another object of the invention to provide an optoelectronic device with controlled optical and electrical feedback and/or input/output isolation. One way to control the optical feedback within the vertical cavity surface emitting laser is to design the mirrors within the lair to have specific reflectivities to obtain optically non-hysteretic, hysteretic, or latching capabilities which will be discussed in further detail. Additionally, an absorbing layer or absorbing mirrors can be placed between the vertical cavity surface emitting laser and the photoactive element. To minimize optical feedback when the interactive component is a phototransistor, the two optical devices may be made to be sensitive to different wavelengths or placed side-by-side so that the optical output of the vertical cavity surface emitting laser does not intersect the photo interactive component. Electrical feedback is provided by a semiconductor layer between the vertical cavity surface emitting laser and the interactive component to prevent minority carrier transport. In an array, complete electronic isolation between the individual devices controls electrical feedback. Advantages realized by these structural features of the invention is that the device may be turned on or off only with optical signals, rather than reversing or stopping the flow of current, thereby facilitating optical computing. Because the device can be designed to have no latching, the device has faster response time because it does not have to overcome positive feedback when switching on and off. Moreover, the device can be designed to adjust the feedback to control variable hysteresis. Arrays using these devices can also be configured as latching devices by allowing optical feedback from the vertical cavity surface-emitting laser to be absorbed in, for example, the base/collector regions of a heterojunction phototransistor or by replacing the heterojunction phototransistor by a thyristor or photodetector/electronic amplifier combination in which case they would function as image memories capable of capturing and storing a two-dimensional image until the array is reset.
Still another object of an embodiment of the invention is to be able to control the wavelengths of the optical signal compared to the wavelength of the output optical signal. When the interactive component is a heterojunction phototransistor, the device can be designed so that the heterojunction phototransistor is sensitive to a wavelength other than that emitted by the vertical cavity surface emitting laser. Moreover, within the heterojunction phototransistor, the bandgaps of the emitter, base, and collector can be adjusted to control the wavelength range over which the phototransistor may be photoactivated. Those features of the surface emitting laser that can be used to control the wavelength of the device include adjusting the bandgap of the laser and adjusting the length of the laser cavity.
It is still another advantage to provide a fundamental optical device suitable for fabrication into large arrays of similar elements in a configuration compatible with parallel optical signal processing. Other advantages achieved are that the device array may be readily fabricated with microfabrication technology, be microscopic in size, and be integrable with other optoelectronic components without being adversely affected by feedback.
Still another object of the invention is to provide an optoelectronic device having an electrical input/optical output. This objective is achieved by the integration of an electroactive component, such as a transistor or dime or other current or voltage controlled components with a vertical cavity surface emitting laser. In particular, both field effect transistors (voltage controlled) and bipolar transistors (current controlled) are examples of such transistors. This vertical or side-by-side geometry provides for an electrical input/optical output component suitable as a building block upon which to build OEICS such as optoelectronic interconnects for computing, electrically driven vertical cavity surface emitting laser arrays for laser-printing, -scanning, and -projection.